Coronary heart disease (CHD) in women increases dramatically at menopause, as estrogen levels drop. Part of the beneficial effect of estrogen on CHD may be through effects on the coronary arteries in ways that protect the heart from ischemic injury. Coronary artery tone and reactivity are regulated by voltage-dependent Ca channels, which allow Ca2+ entry on depolarization, and K+ channels, which when activated cause hyperpolarization. We have provided the first direct measurements in smooth muscle of two types of K+ channels that are likely to play important roles in the response of coronary arteries to estrogen and ischemia: 1) The ATP-sensititive K+ (Katp) channel and 2) The inward rectifier K= (K) channel. Hypoxia, adenosine, and synthetic agents (e.g. lemakalim) activated K channels, and through vasodilation, may be involved in the increase in coronary blood flow to counteract periods of cardiac ischemia. Small increases in extracellular K+, which can occur during ischemia, may activate K channels which may also lead to a beneficial increase in coronary blood flow. Our preliminary results suggest the exciting possibility that estrogen, through alteration in the activity of coronary artery ion channels increases arterial diameter and response to vasodilating signals. We, therefore propose to test the hypothesis that gender and sex steroids, particularly estrogen, affect the reactivity of coronary arteries through their regulation of K+ and Ca channels. Our preliminary evidence to support this hypothesis forms the basis of our 4 Specific Aims: 1) We found that female coronary arteries constrict less to intravascular pressure than male or ovariectomized female coronary arteries from rat. In Specific Aim 1, we will investigate the role of membrane potential (K+ channels), Ca channels, and intracellular free Ca2+ in this response. 2) We found that estrogen but not progesterone dilates coronary arteries and inhibits calcium channels. In specific Aim 2, we shall explore the effects of acute estrogen and other sex steroids on arterial tone, membrane potential, an ion channels. 3) We found that small elevations in external K+ dilate female but not male coronary arteries, suggesting gender and sex steroids on: dilatins and hyperolarizations to external K+, adenosine and K+ channels play a more important role in females. In Specific Aim 3, we shall characterize the effects of gender and sex steroids on: dilations and hyperpolarization to external K+, adenosine and K+ channel openers, and K and K channels. 4) In Specific Aim 4, we will explore the role of the endothelium in all of these responses. We will use a combination of techniques to obtain an integrated picture of coronary artery function: diameter measurements of coronary arteries at different levels of transmural pressure, membrane potential recordings from smooth muscle cells in intact arteries, whole cell and single ion channel measurements in freshly dissociated smooth muscle cells from coronary arteries, and intracellular Ca2+ measurements. The proposed study should provide important insights into the control of coronary artery reactivity by estrogen, other sex steroids and gender. This study should also deepen our understanding of the regulation of coronary artery tone by ion channels, advancing our understanding of possible response mechanism of the female coronary vasculature to ischemia. Finally, the proposed project should suggest new strategies for treatment of coronary heart diseases in women.